1. Field of the Invention
The invention relates to hydraulic fluid control and distribution systems for use on construction loaders, agricultural or industrial tractors, or other mobile implement carrying equipment having a fixed displacement pump working in conjunction with a variable displacement pump having a compensator actuated swash plate. Fluid delivery rates are dictated by the interaction of the pumps and a relief valve.
2. Description of the Prior Art
It is well known to mount two fixed displacement pumps in tandem in a hydraulic circuit and control the output of these pumps through the use of various valving arrangements. However, the use of a fixed displacement pump becomes uneconomical in an application that requires high pressure but negligible flow for any period of time as the fixed displacement pump will continue to attempt to deliver full fluid flow even when only minimal fluid flow is needed by the subject hydraulic system. The inefficiency of having one or more than one fixed displacement pumps running at continuous maximum output when only a minute amount of fluid flow is needed is obvious. Another difficulty encountered is that the pump drive means will be overtaxed unnecessarily as it will be driving the pump or pumps, which will be at high pressure, at a horsepower rate proportionately distorted from the work being performed by the hydraulic system.
A viable alternative to using a fixed displacement pump in a hydraulic system is to use a variable displacement pump of which the stroke can be adjusted to fill the need of either high volume or high pressure as required. Unfortunately, a variable displacement pump is normally an expensive unit when compared to a fixed displacement pump.
Another disadvantage of variable displacement pumps is that they cannot generate the instantaneous pressure sometimes desirable when using construction type earth moving equipment. Operators of construction equipment desire an immediate acting hydraulic system in order to assist in excavating situations where it is felt that penetration of the ground is enhanced by the shock generated when an implement is driven into the ground.
Another apparent disability of the variable displacement pump hydraulic system in an excavating application is that when the operator is operating his digging implement in an area screened from his vision he has difficulty knowing when the implement has penetrated the ground as far as possible. This is because the variable displacement pump will have destroked in order to provide maximum hydraulic pressure and the drive means, usually the vehicle engine, will be providing only nominal horsepower to drive the fully destroked variable displacement pump. For practical purposes the vehicle engine will be close to its idle speed as only minimum horsepower will be needed, yet pressure inside the cylinders operating the digging implements will be very high. At this point the operator will have to guess whether or not the implement has ceased its digging function. In an hydraulic system incorporating a fixed displacement pump the operator will know when the digging implement has reached its maximum potential as the vehicle engine will bog down in response to the high horsepower needed to drive the fixed displacement pump at high pressure.
It is known to provide fixed displacement pumps in series with variable displacement pumps to act as charging pumps for the variable displacement pumps. A fixed displacement pump in this configuration is usually of similar displacement as the variable displacement pump. The drawback of a system having a variable displacement pump charged by a large fixed displacement pump primarily centers around the inefficiency and high cost of providing two pumps of which only one pump is harnessed for useful work.
Therefore, the combination of a fixed displacement and a variable displacement pump in a regulated system such as disclosed in this invention will have the advantage of the instant acting fluid delivery inherent in the fixed displacement pump and the high pressure, low horsepower consuming characteristics available with the variable displacement pump. A combination pump system maximizes performance, efficiency and dependability while minimizing cost.
A specific object in the invention is to provide a hydraulic fluid pump system that is capable of instantaneous fluid flow to the working cylinders.
Another object of the invention is to provide high pressure to working cylinders without wasting horsepower by the unnecessary pumping of fluid. Still another object of this invention is to provide the capability of instant fluid flow and high pressure generation with the unnecessary wasting of horsepower.
Another object of the invention is to provide a combined fixed and variable displacement pump system in which the fixed displacement pump provides fluid flow to the work circuit.